The present invention relates to an extraction unit which comprises, successively in the flow direction of the liquid, a mixer, a pre-settler, and a settler. By arranging several such extraction units in succession, an extraction apparatus is obtained.
In extraction units of the above type, transfer of material between two mutually immiscible or partly miscible liquids is achieved by bringing these liquids into contact with each other in a limited space, i.e. a mixer, by means of a rotating mechanical stirrer, whereafter the dispersion produced in the mixer is directed into another limited space, i.e. a settler, which constitutes part of the same extraction stage, in order to separate the two phases from each other by gravity.
Extraction units are known in which the mixer and the settler are separate vessels, in which case one phase is caused to flow from one extraction unit to another by means of gravity, the extraction units being at different levels, while the other phase is pumped countercurrently from one extraction unit to the next.
However, extraction units are known in which no pump is required between extraction units of an extraction apparatus operating according to the countercurrent principle (J. A. Wiliams, L. Lowes and M. S. Tanner, Trans. Inst. Chem. Engrs. Vol. 36, 464 (1958)). All units are at the same level. The turbine stirrer does not serve as a pump. By keeping the layer of dispersion in the settler at a level below the opening between the mixer and the settler, a pressure gradient is maintained in order to force the dispersion to flow into the settler, since the dispersion has a higher specific gravity than the light phase in the settler. Since the surface of the dispersion in the mixer settles at a level which is below the surface of the light phase in the settler of the adjacent unit, the light phase can flow from there to the mixer of the other unit, i.e., passage of phases between adjacent units is also possible.
In one prior known system, in which the mixer is provided with a pumping turbine stirrer which was originally developed for the treatment of nuclear fuel, the solution is drawn by suction upwards into a hollow turbine shaft and ejected between the blades of the turbine stirrer mounted on the shaft (B. V. Coplan, J. K. Davidson and E. L. Zebroski, Chem. Eng. Progr. Vol. 50 No. 8, p. 403 (1954)). In this case also, the dispersion is led directly from the mixer to the settler.
Recently various pump mixer-settler type extraction apparatus have been given their due recognition in the metallurgical industry. The following principles have been suggested for the system.
In one apparatus the mixer is level-floor vessel in which a turbine has been mounted (D.W. Agers and E.R. Dement, Proceedings International Symposium "Solvent Extraction in Metallurgical Processes", p. 27, Technologisch Institut K.VIV, Antwerpen 1972). The turbine located above the mutual inlet for the light and the heavy phase is structurally a horizontal plate provided with blades on the lower side. The turbine pumps the dispersion through the outlet in the upper section of the mixer and through an expanding trough directly into the settler. In the settler there are vertical perforated partition walls in the vicinity of the inlet of the dispersion in order to retard the flow of the dispersion.
Also known is a pump mixer-settler type extraction apparatus in which the cross section of the mixer is square and in which both the light phase and the heavy phase are directed into the mixer through a suction cylinder fitted at its floor (G. C. I. Warwick, J. B. Scuffham and J. B. Lott, Proceedings International Solvent Extraction Conference ISEC '71, Vol. 2, p. 1373, Society of Chemical Industry, London, 1971). The blades of the turbine are covered by an uniform round plate above and with a level, circular ring below, the opening of the ring being above the opening of the suction cylinder. The height of the suction cylinder has been dimensioned so that the turbine can be mounted in the center of the mixer. The dispersion is forced out through an opening, around the turbine shaft, in the horizontal partition floor, the partition floor delimiting upwardly the mixing chamber, whereafter the dispersion, undivided, flows over this partition directly downward into the settler and substantially on the same level as the dispersion layer in the settler, in which case the dispersion flow is directed by means of one vertical and one horizontal plate.
Also previously known is a pump mixer-settler type extraction apparatus in which two separate turbines have been mounted on the same shaft (T. K. Mattila, Proceedings Solvent Extraction Conference ISEC '74 Vol. 1, p. 169, Society of Chemical Industry, London 1974). The upper turbine improves the mixing effect of the lower pump turbine mounted in the vicinity of the mixer floor, above the inlet of the heavy phase, and gives a pressure impulse for directing the dispersion from the mixer into the settler. In this extraction apparatus the mixer turbine rotates at the level of the dispersion outlet pipe near the center point of the mixer as seen in the vertical direction. The dispersion is fed directly from the mixer into the settler, where the dispersion flow is retarded by stop plates which divide the dispersion. The extraction units have been advantageously graduated in the vertical direction so that light phase flows more easily by gravity into the following unit. The inlet for the light phase is in the upper section of the mixer.
In one extraction apparatus structure (J. Mizrahi, E. Barnea and D. Meyer, Proceedings International Solvent Extraction Conference ISEC '74, Vol. 1, p. 141, Society of Chemical Industry, London 1974), turbines mounted on separate, concentric shafts are used for stirring and pumping, in which case these actions can be regulated independently of each other. The pump turbine has been mounted in the rising cylinder which is above the stirring turbine. Both the mixer and the settler are cylindrical. The phases catering the mixer are directed towards the floor of the mixer, whereas the dispersion emerging from the mixer is fed directly to the center of the settler.